Method of continuously producing polyamide 6 using recycled lactam

ABSTRACT

The invention concerns a method of continuously producing polyamide 6 using recycled lactam from extraction and melt-demonomerizing stages. According to the invention, the cyclic oligomers present in the recycled lactam owing to the addition of ε-caprolactam (fresh lactam) are dissolved in the lactam as a function of the concentration of the oligomers in the product in use, at a dissolution temperature of between 120° C. and 180° C. The solution is then passed to a treatment, the solution preferably being introduced into a closed system and hydrolyzed in the fusible phase under pressure, at water contents of between 3 and 15% and temperatures ranging from 220 to 280° C. With a constant low water content being set, the melt is then fed to the polymerization process in a further pressure stage incorporating a gas chamber (polymerizator).

The invention relates to a multistage method of continuously producing apolyamide 6 using recycled (returned) lactam which is obtained inextraction stages and melt demonomerization stages for reducing the lowmolecular mass fraction in polyamide 6.

It is known that the polycondensation of ε-capro-lactam to polyamide 6is accompanied by the establishment of a chemical equilibrium which isdependent on temperature. At a customary plant reaction temperature of280° C., a water-soluble fraction of approximately 13% becomesestablished.

For the further processing of the polymer, it is necessary to remove theunreacted fraction. Various techniques are known for this purpose. Forexample, granules are produced from the melt at equilibrium, and thesegranules are extracted with water. This removes ε-caprolactam, oligomersand dimers. This technique necessitates subsequent drying of thegranules and working up of the aqueous extraction waters by evaporativeconcentration.

Also known are techniques where unreacted ε-caprolactam, dimers andoligomers are driven off from melt films by means of vacuum or inertgas.

For economic reasons, these recovered products should be returned to thepolymerization process.

There are techniques in which PA 6 extract water concentrates with highor low solids contents are added in lesser or greater proportions to thefresh lactam so that they too can be converted to polyamide 6. Inaccordance with DE-B 25 01 348 and DE-A 27 32 328, polyamide 6 extractwaters concentrated to a solids content of more than 90% and,respectively, to a solids content of 60-70% are polymerized, togetherwith more than the equal amount of fresh lactam based on the solidscontent, with propionic and, respectively, benzoic acid in a reactorwhich is operated essentially at atmospheric pressure.

A disadvantage of this technique is that when polymerizing polyamide 6extract water concentrates with monocarboxylic acids, the polymerizationtimes required are longer than when polymerizing fresh lactam withmonocarboxylic acid. The conversions of ε-caprolactam and of cyclicoligomers obtained, moreover, are approximately 2 to 3% below those offresh lactam polymers, since the cyclic oligomers play essentially nopart in the reaction mechanism. The high water content in the reactionmixture, in conjunction with the unpressurized operation of thepolymerization reactor, leads to an unfavorable reaction course. Thewater is unable to act in the reaction since it evaporates.

It is additionally known that melt vacuum demonomerization processes areoperated such that the demonomerization products drawn off in gas form,i.e. ε-caprolactam and cyclic oligomers, are condensed in fresh lactamand so also subjected to the polymerization (DE-A 28 21 886 and DE-A 2948 865). In the case of similar processes as well, the cyclic oligomers,and especially the cyclic dimers, are not transformed into reactivecompounds.

Also known is the strong catalytic effect of o-phosphoric acid on thepolymerization of ε-caprolactam Geleji, F. et al. "Wirkungsmechanismusverschiedener Katalysatoren bei der Caprolactampolymerisation" Mechanismof action of various catalysts in the polymerization of caprolactam! inFaserforschung und Textiltechnik 13 (1962)6, 282-283! in relation to thepolymerization of cyclic oligomers (Schade, H. and Beckhaus, F. W.:Verfahren zum Polymerisieren von Oligomeren des ε-Caprolactams Processfor polymerizing oligomers of ε-caprolactam!--DD Patent 24 307).Disadvantages of such a polymerization technology are, inter alia:

excessively rapid rise in viscosity and hence unsatisfactoryreproducibility of the desired degree of polymerization, since theo-phosphoric acid acts essentially as catalyst and less as chainregulator

inadequate conversions of ε-caprolactam and cyclic oligomers, and

depolymerization processes in the course of processing of polyamide 6granules of this type.

In order to circumvent these above-mentioned problems, it is common whenreprocessing the aqueous extraction waters to separate off the oligomersby distillation. The oligomers separated off are landfilled, whichpollutes the environment, or else subjected to energy-intensivedepolymerization to give lactam, by processing with phosphoric acid, andthe lactam is subsequently passed to a purification stage. The residuesare landfilled.

For technical reasons and on grounds of economy, polyamide producershave to date concentrated the lactam/oligomer mixture by evaporativeconcentration to a residual water content of ≦2%. At this water content,it becomes impossible to bind the cyclic dimer chemically into thepolymer structure.

The aim of the invention is to ensure reliable and continuous processingin the course of the production of polyamide 6 using recycled lactamand, in particular, to bind cyclic dimers and cyclic oligomers whichhave been introduced by way of recycled lactam into the productionprocess and to process the entire amount into high-grade end products ina closed circuit of materials (without waste).

The object of the invention is to dissolve oligomers present in therecycled lactam in the lactam and to hinder the formation of the cyclicdimer in the course of the polymerization and to convert, in a simplemanner, cyclic dimers of the lactam/oligomer mixture which are presentat the same time from recycled lactam into a reactive compound and toallow them to take part in the production process.

This object is achieved by virtue of the features described.Advantageous embodiments and developments of the invention are alsodescribed.

In accordance with the invention, when using recycled lactam with theaddition of ε-caprolactam (fresh lactam), further processing is precededby the cyclic oligomers present in the recycled lactam being dissolvedin the lactam. In this case, depending on the initial concentration ofthe oligomers in the feedstock, a solution temperature of the oligomersin the lactam of between 120 and 180° C. is established.

For this purpose, for example, the recycled lactam is conveyed from arecycled lactam container by means of a heated pump, is heated by a heatexchanger to preferably 150° C. and is circulated and/or passed to afurther operation.

In the case of the production of polyamide 6 in accordance with theinvention, using recycled lactams, the further processing of therecycled lactam takes place with oligomers dissolved in the lactam, therecycled lactam being introduced, with or without the addition of freshlactam, preferably into a closed system and being treated under pressurein the liquid melt phase by hydrolysis at water contents of from 3 to15% and at temperatures in the range of 220-280° C.

In the course of the hydrolysis of the cyclic oligomers, especiallycyclic dimers, in the reaction mixture, the water content in thispressure stage (dimer treatment stage) is held preferably, in accordancewith the water content of the recycled lactam, at from 6 to 15%.

Under the conditions of the above-mentioned establishment of waterconcentration and reaction temperature, the dimeric ring compound iscleaved open. In this case the cyclic dimer produces aminocaproic acidand linear dimers, which allow trouble-free insertion into a polymerchain.

It has surprisingly been found that about 3% of water must be present inthe lactam/oligomer mixture in order to cleave the cyclic dimer, thewater fraction making a favorable and fully effective contribution tothe reaction under elevated pressure in a closed system. With such asolution, it is possible to employ fresh lactam and recycled lactam andalso recycled lactam, without problems, alone.

The water content of the polymer melt is subsequently adjusted in afurther pressure stage with gas space (polymerizer), by taking off theexcess water, as a function of the desired constant degree of finalpolymerization, to from 0.3 to 1.9%. For this purpose, the pressure inthe gas space of this polymerizer is regulated, preferably by regulatingthe water content and/or the temperature.

The invention will be illustrated by exemplary embodiments:

FIG. 1 shows diagrammatically the treatment of the cyclic dimers and thesubsequent course of the polymerization reaction.

In accordance with the sequence depicted in FIG. 2, undissolved dimersand oligomers present in recycled lactam are dissolved in the lactam.

EXEMPLARY EMBODIMENT 1

For the production of polyamide 6 from recycled lactam, extractionwaters are concentrated to a mean solids content of 93% (long-termfluctuation range 91 to 95%). This so-called recycled lactam consists ofmonomeric lactam, dimers, oligomers and water. When collected in arecycled lactam container, dimers and oligomers are not sufficientlydissolved.

By means of the heated pump 2, the recycled lactam is heated to 150° C.by a heat exchanger 3 and is conveyed in circulation. At the mixingtemperature which is established in the recycled lactam container, alldimers and oligomers are dissolved. To establish a water content >3%, inthis example between 5 and 9%, the pressure-limited regulator valve 5 isemployed. Any water which evaporates is condensed and taken off. Wateris able to evaporate when the water content in the crude lactam is >9%.By means of a metering pump 8, the recycled lactam is pressed into thedimer treatment stage 10 by way of a further heat exchanger 9.

In this heat exchanger 9, the recycled lactam is heated to at least 220°C. The dimer treatment stage 10 comprises a jacketed pressure apparatuswhich is filled with product with no gas phase--and is activelyinsulated through the jacket by means of heat transfer media.

Under these conditions, a polyamide melt having a relative solutionviscosity of from 1.25 to 1.35 (solution viscosity based onpolymer/sulfuric acid solution with 1 g of polyamide in 100 ml of 96%strength acid in comparison to pure 96% strength sulfuric acid) isobtained after the dimer treatment stage.

In order to be able to produce a polymer having constant and relativelyhigh viscosities, the physically dissolved water which is present inexcess must be driven off from the melt in a polymerizer down to arequired water content which is dependent on the desired end viscosity.

A valve 11 ensures that the pressure remains set at above the boilingpoint of the product in the dimer treatment stage 10. By way of anevaporator 12, the product passes into the polymerizer 13, which isoperated under pressure at a water concentration of 1.7%. The evaporatedwater leaves the system via a reflux column, from which the lactampresent in the ascending steam runs back into the polymerizer 13 again.

After the polymerizer 13, the polyamide passes via valve and evaporatorinto a polyamide post-condenser 18. In the bottom part of thispost-condenser 18 the PA melt is cooled to 240° C.

Via a metering pump 20, the polyamide leaves the process with a solutionviscosity of 2.5 and a water-extractable fraction of 9.9%.

EXEMPLARY EMBODIMENT 2

In a polymerization plant for the production of polyamide 6, alactam/oligomer mixture is employed which originates fromdemonomerization stages and accordingly contains virtually no water.

In order to prevent precipitation of the oligomers, this mixture isheated by means of heat exchanger 3 to 150° C., as a result of which thedimers and oligomers dissolve. This solution can be transported withoutproblems over relatively large distances without any blocking of thepipelines as a result of precipitated oligomers.

The lactam/dimer/oligomer solution is heated as in Exemplary Embodiment1 in a further heat exchanger 9, but in this case to not less than 240°C., and is then introduced into the pressure stage of the invention, thedimer treatment stage 10, where it is treated. Between heat exchangerand dimer treatment stage, water is fed in by way of the valve 7 and themelt is adjusted to a water content of 6% of physically dissolved water.The cyclic oligomers, and especially dimers, are reactively cleaved inthe pressure stage by hydrolysis.

The subsequent course of polymerization is as described in ExemplaryEmbodiment 1. The resulting polymer has a solution viscosity of 2.52 andan overall extractables content of 9.9%.

EXEMPLARY EMBODIMENT 3

In a polymerization plant, polyamide 6 is produced from recycled lactamand fresh lactam.

For this purpose the lactam/oligomer mixture, which is recovered fromdemonomerization stages, is admixed with an approximately equal amountof fresh lactam. In these mixing proportions, the oligomers dissolve inthe lactam already at about 120° C. Heating and further treatment of thepolymeric melt are performed as described in Exemplary Embodiment 1. Theinfeed of water to a water content of 6% takes place as set out inExemplary Embodiment 2. The resulting polymer has a solution viscosityof 2.61 and an overall extractables content of 9.7%.

We claim:
 1. A method of continuously producing polyamide 6 usingrecycled lactam from extraction stages and melt demonomerizationstages,which comprises dissolving the cyclic oligomers present in therecycled lactam, with the addition of ε-caprolactam (fresh lactam), inthe lactam as a function of the concentration of the oligomers in thefeedstock and at a solution temperature of between 120 and 180° C. andthen passing the solution to a treatment stage in which the solution isintroduced into a system and is treated in the liquid melt phase in afirst pressure stage by hydrolysis at water contents of from 3 to 15%and at temperatures in the range from 220 to 280° C. and the liquid meltphase is subsequently passed to the subsequent polymerization processwith establishment of a constant lower water content in a furtherpressure stage with gas space.
 2. The method as claimed in claim 1,wherein recycled lactam is conveyed by means of a heated pump from arecycled lactam container and is heated by a heat exchanger and iscirculated.
 3. The method as claimed in claim 1, wherein the watercontent in the liquid melt phase in the course of the hydrolysis of thecyclic oligomers and dimers in the pressure stage is held, in accordancewith the prevailing water content of the recycled lactam, at from 6-15%.4. The method as claimed in claim 1, wherein the water content in thepolymeric melt is adjusted in the further pressure stage with gas space,as a function of the desired degree of final polymerization, at from0.3-1.9% by taking off the excess water.
 5. The method as claimed inclaim 4, wherein the water content in the liquid melt phase in thecourse of the hydrolysis of the cyclic oligomers and dimers in thepressure stage is held, in accordance with the prevailing water contentof the recycled lactam, at from 6-15%.
 6. The method as claimed in claim5, wherein recycled lactam is conveyed by means of a heated pump from arecycled lactam container and is heated by a heat exchanger.
 7. Themethod as claimed in claim 6 in which the solution is introduced into aclosed system first pressure stage.
 8. The method as claimed in claim 6in which the recycled lactam is heated by the heat exchanger to 150° C.9. The method as claimed in claim 2, wherein the water content in theliquid melt phase in the course of the hydrolysis of the cyclicoligomers and dimers in the pressure stage is held, in accordance withthe prevailing water content of the recycled lactam, at from 6-15%. 10.The method as claimed in claim 9 in which the recycled lactam is heatedby the heat exchanger to 150° C.
 11. The method as claimed in claim 2 inwhich the recycled lactam is heated by the heat exchanger to 150° C. 12.The method as claimed in claim 1 in which the solution is introducedinto a closed system first pressure stage.